1. Field of the Invention
The invention relates generally to the global positioning system (GPS) and specifically to methods and devices that allow a user to reduce GPS navigation errors.
2. Description of the Prior Art
The global positioning system (GPS) is a United States funded satellite system consisting of sixteen to twenty-four satellites in a constellation that beams highly accurate timed signals to earth. GPS receivers can process these signals from anywhere in the world and provide a user with position information that can be accurate to within twenty meters. Each GPS satellite moves through a known orbit and the time that individual GPS satellite transmissions take to reach a user's position are used in solving a triangulation problem. The individual signal paths from each satellite to the user's position must pass through the troposphere and ionosphere. Position solution errors can occur when there is not accurately accounted for radio signal propagation delays caused by the troposphere and ionosphere. The ionosphere introduces its maximum delays about 2:00 PM local time and has practically no effect at night. Thus, the local time may be important to a position solution. The troposphere can introduce delays that are a function of barometric pressure, temperature, humidity and other weather variables. Variations in the actual orbit of each GPS satellite can also have an impact on position solution accuracy. Drifts in the clock of each GPS satellite will also impact user position solution accuracy. Service bureaus and agencies have been established to sell or otherwise provide differential data. Such data range from real-time local information to sometimes very complex mathematical models based on long-term observations. US Government agencies issue orbit correction information on a satellite-by-satellite basis.
Dual frequency carrier GPS receivers continuously track P-code L1 and L2 carriers of a GPS satellite to generate accumulated delta-range measurements (ADR) and at the same time track L1 C/A-code to generate code phase measurements. Each carrier is modulated with codes that leave the GPS satellite at the same clock time. Since the ionosphere produces different delays for radio carriers passing through it having different radio frequencies, dual carrier receivers can be used to obtain real-time measurements of ionospheric delays at a user's particular position. (L1 is typically 1575.42 MHz and L2 is typically 1227.6 MHz.) The L1 and L2 ADR measurements are combined to generate a new L1 ADR measurement that has an ionospheric delay of the same sign as the ionospheric delay in the L1 pseudorange. Accurate ionospheric delay figures, if used in a position solution, can help produce much better position solutions. Without such real-time ionospheric delay measurements, mathematical models or measurements taken by third parties (which can be old) must be used instead. The communication of this information to a user's site can be costly and require wide communications channel bandwidths.
Commercial and private users have been able to make use of GPS satellite transmissions even though they use so-called "unauthorized" receivers. "Authorized" receivers are able to receive special information (P-code) that can remarkably improve position solution accuracy. Since an enemy could use GPS to fly and target missiles and artillery shells to better than one meter of accuracy, the United States Department of Defense occasionally engages a selective availability (SA) mode, spoofing and encryption of the GPS signals that deliberately introduce solution errors into user receivers that lack an authorization code needed to see through the masking. A "codeless" unauthorized GPS receiver is able to cross-correlate the L1 and L2 signals and extract enough information to measure the ionospheric delay (although not enough to overcome SA or to read P-code encrypted by Y-code to secure transmissions.)
With differential GPS, a stationary reference receiver is placed at a very accurately known point location. The reference receiver generates corrections which are sent to a transmitter, which in turn broadcasts the corrections to users within the area of the transmission broadcast. A differential GPS user receives these corrections through a radio/modem and applies them to the direct GPS measurements. This gives the user a position measurement of very high accuracy, e.g., from one meter to ten meters.
However, as a user's receiver is moved away from a reference receiver, the corrected accuracy will gradually deteriorate. Thus, the region of usefulness is finite. By employing a network of reference stations, the accuracy achievable by users near a reference station can be had throughout a region of several hundred miles.
While advantages of differential GPS networks have been well known, the basic problems to realization of them are many. A means of reducing the communications volume between the reference stations is needed in order to keep the cost of service down and thus to be competitive. A means of reducing the communications volume between the reference station and the user population is needed. Also needed are effective techniques and algorithms that enable a minimum number of reference stations to be employed. Effective techniques and algorithms are needed that provide high accuracy throughout a large operating region, for example, worldwide in a band of latitudes that include most of the populated areas.
A paper written by Alison Brown, titled "Extended Differential GPS," published by Navigation: Journal of the Institute of Navigation, in Vol. 36, No. 3, Fall 1989, pp. 265-285, presents an extended differential GPS concept that uses a network of differential GPS stations. A differential GPS message can be computed that provides corrections for the different components of the pseudorange error. This is reported to extend differential GPS navigation accuracy to one thousand nautical miles from a master differential station.